Move the layout tree from GC allocation to refcounted ownership so
removed layout and paint subtrees are destroyed synchronously instead
of waiting for the next GC sweep. This dramatically reduces GC memory
usage peaks after layout tree churn and makes it easier for memory use
to fall back after large document updates.
Update layout factories, tree traversal, SVG layout node creation,
paintable back-pointers, and pseudo-element layout links to use RefPtr
ownership.
Make display: contents follow the same shape as Blink and WebKit: the
element itself does not create a layout node, and its children are
flattened into the nearest layout parent. Wrap direct non-whitespace
text in an anonymous inline node when the boxless element contributes
inherited style to that text.
Use an internal inline wrapper for display: contents pseudo-elements
so generated content can still participate in layout, painting, hit
testing, and pseudo-element queries. Keep CSSOM reporting the computed
display value from the pseudo style, not the internal wrapper.
Remove the retained out-of-tree layout node list and its testing hook,
since the flattened model does not need a side owner for boxless
elements. Add coverage for inherited text style, dynamic insertion
order, pseudo-element hit testing, and computed style queries.
Build a hit-test display list while recording paint output. Use it as
source of truth for point hit testing instead of recursively walking the
paintable tree in reverse paint order.
The retained list records target paintables, visual context indices,
border radii, caret rects, and line metadata needed by hit testing. It
also keeps a spatial index so point queries inspect nearby items before
checking containment in paint order.
Refresh scroll state before hit testing so visual context transforms use
current scroll offsets. Add text tests for rounded hit regions and
selection across non-text content.
This is preparation for moving the Paintable tree from GC allocation to
ordinary ref-counting. Paintables need tree ownership semantics that
match TreeNode, but their lifetime is about to stop being controlled by
the JS heap.
Add a small ref-counted tree helper and unit tests for ownership,
sibling links, removal, destruction, and preorder traversal. No
Paintable code uses it yet; the behavior change happens in the next
commit.
